A plurality of procedures are known for the photopolymerization of vinyl monomers by free radical mechanisms. Such procedures involve radiation-induced processes which generate free radicals by the use of high energy ionizing radiation from radio-active sources, electron beams, X-rays or quartz UV-radiation, as described typically in U.S. Pat. No. 3,926,756 and U.K. Patents Nos. 1,279,108 and 1,437,281 and photochemically-induced processes, as described typically in U.K. Patents Nos. 567,778, 1,053,097 and 1,332,247.
More recently, U.K. Patents Nos. 1,310,729 and 1,379,088 have described a combination photochemically-and chemically-induced process. In this prior art, polymerization is carried out at pH values in the range of about 8 to 13, which is unsuitable for many vinyl monomers, such as acrylamide, acrylic-type and cationic monomers. In some instances, for example, in the case of acrylic or maleic alkyl (C.sub.1 to C.sub.4) esters, the high pH increases the hydrolytic decomposition of the monomers to lower alcohols, which may act as chain transfer agents and result in molecular weight limitations.
The specific examples contained in these prior art patents indicate a rather high maximum temperature of the polymerization mass, namely from about 115.degree. to about 150.degree. C. These high temperatures can bring about the risk of volatilization of materials in the solution being polymerized and form toxic vapors as well as promote localized cross-linking through chemical or free-radical routes.
In the prosecution of the parent application, there was cited U.S. Pat. No. 3,912,607 (Communal). This reference discloses a process for obtaining high molecular weight water-solution acrylic polymers and copolymers by polymerizing monomer solution of pH at least 8 using a photopolymerization initiator and subjecting the mixture of luminous radiation for 30 secs to 15 mins. The distinctions of the present invention over this art will become apparent below.